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Delving into Black Holes

Author: Deepthy Ajith K

Editor: Lucia Kajganic


1. History of black holes

2. What are black holes?

3. What is a supermassive black hole?

4. Understanding Accretion

5. Physical properties of supermassive black holes

6. How are black holes and supermassive black holes formed?

7. Characteristics of a black hole

8. How do we know black holes exist?

9. What is Hawking radiation?

10. And finally, let’s talk about Sagittarius A*



Considered to be an astronomy pioneer, English clergyman John Mitchell was the first to propose the notion of a massive body – so massive that even light could not escape. He referred to these bodies as ‘dark stars’ in a letter published in November of 1784. 

While Fred Hoyle and W.A Fowler had proposed the existence of hydrogen burning supermassive stars having a mass of about 105 – 109 M☉, as early as 1963, to explain the compact dimensions and high energy output of quasars, Appenzeller and Fricke concluded that a non-rotating 0.75×106 M☉ supermassive star “cannot escape collapse to a black hole by burning its hydrogen through the CNO cycle”.

Maarten Schmidt’s investigation of the radio source 3C 273 in 1963 found the Supermassive black holes. Its red-shifted hydrogen emission lines indicated that the object was moving away from Earth. Four more such sources were identified by 1964.

The formulation of black hole thermodynamics was later undertaken by James Bardeen, Jacob Bekenstein, Carter, and Stephen Hawking in the early 1970s. These laws described the behaviour of black holes, but analogous nature to the laws of thermodynamics was completed only in 1974 when Hawking demonstrated that the quantum field theory implied that black holes should radiate like a black body (an idealised physical body that absorbs all incident electromagnetic radiation) with a temperature that is proportional to the surface gravity of the black hole, an effect now known as Hawking radiation.

Cygnus X-1 was the first black hole identified in 1971 by independent researchers.

Miyoshi et al. (1995) observing Messier 106, demonstrated that the emission from an H2O maser (microwave amplification by stimulated emission of radiation) was from a gaseous disk in the nucleus that orbited a concentrated mass of 3.6×107 M☉, constrained to a radius of 0.13 parsecs. Their ground-breaking research noted a supermassive black hole was the sole viable candidate as a swarm of solar mass black holes within a radius this small would not survive for long. This observation provided the first confirmation of supermassive black holes.

The Event Horizon Telescope collaboration released the first horizon-scale image of a black hole, in the centre of the galaxy Messier 87 on April 10, 2019.



A region of space-time where exceptionally strong gravity prevents even light from escaping, a black hole is extremely dense and acts like an ideal black body, as it reflects no light. 

Black holes are among those astronomical objects that have undergone gravitational collapse, leaving behind spheroidal regions of space having a boundary of no escape, called the event horizon.

Einstein’s theory of general relativity predicts that a sufficiently dense mass can deform space-time to form a black hole. 




As the name suggests, these are massive black holes. Supermassive black holes, the largest type of black holes, are classically defined as black holes with a mass above 0.1 million to 1 million times the mass of the Sun (M☉). Observational evidence indicates that almost every large galaxy has a supermassive black hole at its centre.



Accretion is defined as the accumulation of particles into a massive object, by gravitationally attracting more matter (typically gaseous), in an accretion disk. Most astronomical objects, such as galaxies, stars, and planets, are formed by accretion processes.

When the accreting object is a neutron star or a black hole, its proximity to the compact object causes the gas in the inner accretion disk to orbit at very high speeds. This results in friction heating the inner disk to temperatures at which vast amounts of EM radiation are emitted – this process is one of the most efficient energy-producing processes known as up to 40% of the rest mass of the accreted material can be emitted as radiation.

Accretion of interstellar gas into supermassive black holes is what’s responsible for powering active galactic nuclei and quasars.





The physical properties of supermassive black holes provide a distinct basis to distinguish them from lower-mass classifications. 

☆ Supermassive black holes can grow only up to a certain upper limit. Ultra-massive black holes appear to have a theoretical upper limit of around 50 billion solar masses, and anything above this slowing growth down to a crawl (it starts around 10 billion solar masses). This causes the unstable accretion disk surrounding the black hole to coalesce into stars that orbit it.

☆ The tidal forces in the vicinity of the event horizon are significantly weaker compared to those of stellar mass black holes. This force at the event horizon is inversely proportional to the square of the black hole’s mass. 

☆ Since the volume of a spherical object (such as the event horizon of a non-rotating black hole) is directly proportional to the cube of the radius, it can be seen that higher the mass of black holes, the lower the average density is.






Gravitational collapse takes place when an object’s internal pressure is unable to resist its own gravity. This typically occurs in stars when a star has insufficient fuel to maintain its temperature or because it receives additional matter without its core temperature being raised. Both cases indicate that the star’s temperature is not sufficiently high to prevent it from collapsing under its own weight.

Some scientists think supermassive black holes formed at the same time as the galaxy they are in while others postulate they might come from direct collapse of dark matter with self-interaction. It has also been suggested that black holes with masses of ~105 M☉ may have formed from the direct collapse of gas clouds in the young universe.

A stellar black hole is a black hole that forms by the gravitational collapse of the centre of a heavy star. According to present theories, the formation of stars in the early universe may have resulted in massive stars which underwent gravitational collapse to produce black holes that could have served as seeds for supermassive black holes assumed to be found in the centre of most galaxies. 

Black holes following their formation can continue to grow by absorbing gas and interstellar dust from its surroundings. This is a possible path for how supermassive black holes may have formed. 

Black holes can also merge with other objects such as stars or even other black holes. This is assumed to be important in the early growth of supermassive black holes of millions of solar masses (M☉), which may have formed by absorbing other stars and merging with other black holes. 




☆ Event horizon

The Event Horizon is a defining feature of black holes. This is the boundary in space-time that doesn’t allow anything to escape from inside it, even light. Matter can pass inwards i.e towards the black hole but not outwards. This boundary is called the event horizon because if an event does occur within it, information pertaining to it cannot escape out to an external observer, rendering it impossible to determine whether the event occurred or not. 

☆ No-hair theorem

The no-hair theorem postulates that once it achieves a stable condition following formation, a black hole has three independent physical properties, namely mass, electric charge, and angular momentum. If true, any two black holes that possess the exact same values for mass, electric charge and angular momentum, are indistinguishable from one another.

☆ Singularity

A gravitational singularity is a region where the spacetime curvature becomes infinite. General relativity suggests that such a point exists at the centre of a black hole. The singular region has zero volume and contains all the mass of the black hole solution suggesting that it may be thought of as having infinite density as density is equal to mass upon volume.

☆ Photon sphere

The photon sphere is essentially a spherical boundary of zero thickness. Photons that move tangentially to this sphere would be trapped in a circular orbit about the black hole. 

Unlike the event horizon, light can escape from the photon sphere. But light that crosses the photon sphere on an inbound trajectory will be captured by the black hole. In short, the only light that can reach an observer is that which must have been emitted by objects between the photon sphere and event horizon. 

☆ Ergosphere

General relativity predicts that any rotating mass will tend to slightly “drag” along the spacetime immediately surrounding it, a process known as frame-dragging. A region of spacetime surrounds rotating black holes, where it is impossible to stand still. This region is called the ergosphere and is a volume bounded by the black hole’s event horizon and the ergo surface.

In fact, this effect is so strong near the event horizon that an object would have to move faster than the speed of light in the opposite direction to just stand still.

☆ Innermost stable circular orbit (ISCO)

The smallest marginally stable circular orbit in which a test particle can stably orbit a massive object in general relativity is called the innermost stable circular orbit (often called the ISCO). It plays an important role in black hole accretion disks since it marks the inner edge of the disk.





As black holes themselves don’t emit any electromagnetic radiation other than the hypothetical Hawking radiation, the search generally relies on indirect observations. 

The presence of a black hole can be inferred through its interaction with other matter and electromagnetic radiation. Quasars, some of the brightest objects in the universe, are formed when matter falls onto a black hole forming an external accretion disk heated by friction. 

Astronomers established that the radio source known as Sagittarius A*, at the core of the Milky Way galaxy, contains a supermassive black hole of about 4.3 million solar masses by observing the orbits of stars orbiting a black hole to determine the black hole’s mass and location.

On 14 September 2015, the LIGO (The Laser Interferometer Gravitational-Wave Observatory) made the first-ever successful direct observation of gravitational waves. This observation provides the most concrete evidence for the existence of black holes to date. 



In 1974, Hawking predicted that black holes emit small amounts of thermal radiation at a temperature ℏc3/(8πGMkB), an effect today known as Hawking radiation – named after the physicist who developed a theoretical argument for its existence.

According to Wikipedia, Hawking radiation is thermal radiation that is theorised to be released outside a black hole’s event horizon because of relativistic quantum effects. It reduces the mass and rotational energy of black holes and is therefore also theorised to cause black hole evaporation. As the radiation temperature is inversely proportional to the black hole’s mass, micro black holes are predicted to be larger emitters of radiation than larger black holes and should dissipate faster.

Assuming Hawking’s theory of black hole radiation to be true, black holes are expected to shrink and evaporate over time as they lose mass by the emission of photons and other particles.

Since Hawking’s publication, many others have verified the result through various approaches.

The supermassive black hole at the centre of the Milky Way galaxy is called Sagittarius A*. As early as 1971, Donald Lynden-Bell and Martin Rees hypothesised that the centre of the Milky Way galaxy would contain a massive black hole. Subsequently,  Sagittarius A* was discovered and named on February 13 and 15, 1974, by astronomers Bruce Balick and Robert Brown.

They discovered a radio source that was dense and immobile owing to its gravitation that emitted synchrotron radiation (electromagnetic radiation emitted when charged particles travel in curved paths.). This was the first indication that a supermassive black hole existed in the centre of the Milky Way.

Located near the border of the constellations Sagittarius and Scorpius, visually close to the Butterfly Cluster (M6) and Lambda Scorpii, Sagittarius A*  is a bright and very compact astronomical radio source. Based on mass and increasingly precise radius limits, astronomers have concluded that Sagittarius A* must be the Milky Way’s central supermassive black hole with the current value of its mass considered to be 4.154±0.014 million solar masses. 

On May 12, 2022, astronomers released the first image of the accretion disk around the horizon of Sagittarius A* using the Event Horizon Telescope. It was produced using a world-wide network of radio observatories made in April 2017, confirming the object to be a black hole. 



[1] “What is a black hole?”,They%20are%20invisible.

[2] “Black Hole” Wikipedia

[3]”Supermassive black hole” Wikipedia

[4] “Black Holes: Everything You Need To Know”

[5]”Hawking Radiation” Wikipedia

[6] “Sagittarius A*” Wikipedia*

[7] “Accretion” Wikipedia

[8] “Maser” Wikipedia,at%20Columbia%20University%20in%201953.

[9] ”Black Body” Wikipedia

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